Method for real-time communication between a number of network subscribers in a communication system using ethernet physics, and a corresponding communication system using ethernet physics

ABSTRACT

For real-time communication between a number of network subscribers in a communication system using Ethernet physics, the slave units are synchronized to the master unit in that each slave unit is clocked via a respective timer with a predetermined overall cycle time, which timer is set cyclically by the reception of respective slave-specific synchronization information which is determined by the master unit. In this case, each timer in a slave unit automatically starts a new cycle once the predetermined overall cycle time has elapsed, even in the absence of the respective synchronization information. Access control for the transmission mode and reception mode between the network subscribers is provided using a timeslot access method.

FIELD OF THE INVENTION

The invention relates to a method for real-time communication between anumber of network subscribers in a communication system using Ethernetphysics, wherein a master unit and one or more slave units communicatewith one another by means of messages which are transmitted via thenetwork.

BACKGROUND OF THE INVENTION

As widely differing technical systems are increasingly networked, thereis a growing requirement for standardized structures in industry. Inthis context, it is also desirable to be able to couple any desiredappliances locally. In order to provide open systems for networking, itis necessary to provide simple and cost-effective communicationmechanisms which enable industrial appliances to have a networkingcapability. In particular, this requirement also exists in the contextof the coupling of drive components, such as drive controllers, powersections and transmitters, for numerically controlled machine tools androbots, in which a number of interpolating axes must be operatedsynchronously.

In present-day high-performance drive systems, the interfaces totransmitters and power sections are in the form of analog signalinterfaces. This, however, involves considerable restrictions relatingto the spatial distribution capability, since the susceptibility tointerference from EMC effects (EMC stands for electromagneticcompatibility) increases with the cable length. If the performancerequirements are low, proprietary company serial digital transmissionsystems are generally used. Where the performance requirements are high,the communication between the drive controller and the movementcontroller is provided by proprietary-company serial data transmissionsystems.

Recently, with regard to the requirement to provide industrialappliances with a networking capability the ETHERNET (data transmissionrate 10 Mbps), in particular the FAST ETHERNET (data transmission rate100 Mbps—IEEE Standard 802.3-1998) data transmission technology which isknown from office technology, has been becoming increasingly important.This is due to the fact that this development represents an undefinedStandard with regard to compatibility and, furthermore, is available atlow cost, since appropriate interface hardware is being produced inlarge quantities, owing to the widespread use in the field of personalcomputers. Furthermore, ETHERNET networks are already widely used inmany organizations, so that a widely extended infrastructure can alreadybe made use of. These arguments all favor the use of the ETHERNET in thefield of automation technology as well. ETHERNET is generally used inthe field of Local Area Networks LAN, with the most widely usedtransmission protocol being TCP/IP (Transmission ControlProtocol/Internet Protocol).

Accordingly, the IEEE Standard 802.3, CSMA/CD (Carrier Sense MultipleAccess/Collision Detect) is generally used as the access method. In thismethod, all the network subscribers have equal priority, and any networksubscriber is allowed to send a message on the network, generally a bussystem at any time. However, a problem occurs in this case when two ormore subscribers are sending a message at the same time. In thissituation, a collision is identified, and each subscriber involved isthen assigned a waiting time, which is defined randomly, before anotherattempt to send the message is made. The term statistical access methodis therefore used.

The requirements for the performance of communication systems forautomation technology are particularly stringent, for example whencoupling drive components. When interchanging data between transmitters,power sections and a drive controller, the data transmission time, whichis included in the control loop as a dead time, is a particularlyimportant parameter. The shorter this dead time, the better the dynamicresponse which can be achieved by the control system.

The connection between movement controllers and drive controllers isalso sensitive to dead times, since a control loop is also closed viathis connection. There is thus a problem in particular in the datatransmission time for serial communication systems, which can be solvedonly by an appropriately fast system with a real-time capability, thatis to say a deterministic system. However, the fact that communicationusing ETHERNET networks does not ensure a determined time response runscounter to their use for automation technology. The ETHERNET Standardtherefore does not offer the technical preconditions for real-timecommunications.

IEEE Standard 802.3 defines a message frame which is not suitable forthis purpose. Since, however, the components for physical ETHERNETsignal transmission are independent of the Standard protocol form, thedeveloper has freedom to choose the protocol form in which the data areto be transmitted. Only Layer 1 (the physical layer) is adopted fromIEEE Standard 802.3.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a transport protocolwhich allows real-time communication while including the widely usedEthernet physics. According to the present invention, this object isachieved by a method for real-time communication between a number ofnetwork subscribers in a communication system using Ethernet physics,wherein:

-   -   a master unit and one or more slave units communicate with one        another by means of messages which are transmitted via the        network;    -   the messages are interchanged cyclically with equidistant        sampling times, in that each slave unit is synchronized to the        master unit by means of a common timebase; and    -   access control for the transmission mode and reception mode is        carried out between the network subscribers using a timeslot        access method.

Since the aforementioned applications require both high-precisioncompliance with the real-time condition and a high level of security andreliability in the transmission, the standardized transmission layer 2(message frame and access method) of the (Fast) Ethernet, which does notsatisfy these requirements, is completely redefined by a new messageframe and a new access control method, and the Ethernet physics are thusused as the basis for real-time communication between, for example,drive components. Both the communication between the control unit andthe transmitters and power sections, and the connection to a movementcontroller, can be provided in this way.

With regard to synchronization between the master and slave units, ithas been found to be advantageous for the slave units to be synchronizedto the master unit by each slave unit being clocked via a respectivetimer with a predetermined overall cycle time, which is set cyclicallyby the reception of a respective slave-specific synchronizationinformation item, which is determined by the master unit. A master-slavecommunication architecture is thus used. In order to allow cyclic datainterchange with identical sampling times to be provided, a common timebase is produced for the master and all the slaves. The slaves aresynchronized to the master by means of specifically defined messages,defined in time, from the master to the slaves, and individuallyconfigured timers in the slaves. User data messages and specificsynchronization messages, which contain the respective synchronizationinformation, can be transmitted in this way. Alternatively, thesynchronization information can also be integrated in a specificallydefined user data message. The stability of the communication system canbe further enhanced if each timer in a slave unit automatically starts anew cycle once the predetermined overall cycle time has elapsed, even inthe absence of the respective synchronization information.

A timeslot access method, which is initialized by the master in thenetwork and allows optimum dead-time data transmission, is used forcyclic data transmission for the transmission and reception modes.Accordingly, messages can be monitored precisely for premature ordelayed transmission, or for transmission subject to interference. Forthis purpose, only the master unit, has transmission authorization onthe network for initialization, and reports to each slave unit which hasonly response authorization, via an appropriate slave-specific message.In addition to the overall cycle time, the time slots within the overallcycle time determine when the respective slave unit will receivemessages from the master unit and the times at which it should send itsmessages. It has been found to be advantageous if each slave unit istold the respective synchronization time in the initialization phase.

Simultaneous and equidistant sampling for a control system can beachieved by storing instantaneous values in each slave unit at a commontime, in particular at the start of a cycle.

In a preferred embodiment of the method according to the presentinvention, monitoring information is provided in each message which istransmitted by the master unit to a slave unit, by means of whichsecurity or safety functions which are provided in the slave unit can beactivated directly via a second initiation channel.

The user data can be transported in a message frame which, in additionto slave addressing and message length information, provides protectionof the data integrity by means, for example, of a CRC checksum andfurther security and safety related data areas. The data in the messageframe can be evaluated not only by an application processor, but also bya communication module, which allows a second initiation channel. Forthis purpose, each slave unit sends a life signal with each message tothe master unit. The absence of this signal indicates that the slavesystem has crashed, and the master unit stops this slave unit in acontrolled manner by means of the second initiation channel (without theassistance of the slave unit). Furthermore, in the user data area of itsmessage to the slave unit, the master unit can initiate a function inthe slave unit, and can initiate this simultaneously by means of asignal in the second initiation channel. Two-channel initiation is thusachieved, which is an essential requirement for certain security orsafety applications. The master unit can also send a master life signalin each of its messages to the slave units. In the absence of thissignal, all the slaves react by stopping their own functions in acontrolled manner.

Although the transmission technology based on the Ethernet Standardallows only point-to-point connections, it is also possible to formnetworks by using network nodes (which are referred to as HUBs) in(fast) Ethernet networks by a number of network subscribers, or eachnetwork subscriber, having one circuit part in order to form networknodes. The circuit part is used for passing on the messages in thedirection of another master unit or further slave units, whereincommunication between network subscribers via network nodes likewisetakes place as claimed in one of the procedures described above.

Furthermore, separate transmitting and receiving lines between twonetwork subscribers can be used simultaneously. This is done, forexample, based on the rule that all the slave units transmit only in thedirection of the master unit, and receive only messages directed fromthe master unit. This means that a message from the master in thedirection of a slave and a message from a slave in the direction of amaster can be transmitted simultaneously. This allows for full-duplexoperation.

Real-time communication can be achieved on the basis of a communicationsystem using Ethernet physics by means of the procedure described above.In this case, hierarchical networks can also be produced by means ofpoint-to-point connections, connected via network nodes, using Ethernetphysics for carrying out real-time communication in relatively largenetwork topologies. This is also suitable for networking or coupling adistributed drive system, in that a first communication system comprisesa numerical movement controller as the master unit and at least onecontrol unit as the slave unit. Each control unit is used as the masterunit for a further communication system, which has at least one powersection for driving a motor and an associated transmission system asslave units.

Security and safety applications can also be provided since thecommunication between the drive components, such as a control unit,transmitter, power sections and movement controllers, is accomplished bymeans of an existing high-performance transmission system from theoffice communication field, by means of a completely new protocol,master-slave synchronization and a timeslot access method, for areal-time capability.

The use of the (fast) Ethernet transmission technology including themethod according to the present invention thus results, inter alia, inthe following advantages:

-   -   low-cost line drivers, since large quantities of these are used        in office communication technology—a proven technology which is        also used in industry;    -   the line drivers allow any desired protocol and full-duplex        operation;    -   better cable material can be used than for multi-core analogue        signal cables;    -   the distances between the components can be greater than when        using analog signal cables;    -   shorter dead times can be achieved by means of high transmission        performance levels of up to 100 Mbps with fast Ethernet; and    -   even complex networks can thus also be constructed.

DRAWINGS

Further advantages and details of the present invention are describedbelow in conjunction with the figures, in which:

FIG. 1 shows an outline illustration of a movement controller and adrive appliance having two axes, based on the communication system ofthe invention;

FIG. 2 shows a timing diagram of the timer synchronization in a slave;

FIG. 3 shows an illustration of the timeslot access method;

FIG. 4 shows an illustration of one possible message frame;

FIG. 5 shows an illustration of one possible function code within amessage frame; and

FIG. 6 shows an outline illustration for a second initiation channel forsecurity and safety applications.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a movement controller NC and of a driveappliance with two axes, which are coupled by means of a firstcommunication system KOMSYS1 based on Ethernet physics. The drivecontroller R is connected to its power sections A1, A2 and transmittersS1, S2 by means of its own independent communication system KOMSYS2.Each power section A1 and A2 is used to drive the respective motor M1,M2, to which respective sensors S1, S2 in the form of transmittersystems are in turn connected, in order to detect the current, rotationspeed and position and orientation information required for position andorientation control.

With regard to the first communication system KOMSYS1, the movementcontroller NC represents the master unit, which communicates with anumber of controllers R (FIG. 1 shows only one control group, for thesake of clarity) as slave units. In the second communication systemKOMSYS2, the controller R forms the master unit, while the actuators A1,A2 and sensors S1, S2 represent the slave units.

The network subscribers are connected via fast Ethernet line drivers PHYwithin each network subscriber. By way of example, the line drivermodules PHY of the fast Ethernet are used for physical datatransmission, on the basis of a copper cable having at least four cores,or a two-conductor optical waveguide.

In order to network different communication systems, all the networksubscribers, or some of them (in the present case, the movementcontroller NC and the actuators A1, A2) have circuit parts HUB which areconnected downstream from the line driver modules PHY and are used topass on the messages in the direction of the master or further slaves.It is thus possible to form a hierarchical network, as shown in FIG. 1.

The messages are passed via the line drivers PHY and any network nodesHUB to the respective protocol modules Kom, which process the messageprotocol and in which the timeslot access method according to theinvention is used. To this end, the principle of synchronization betweena master and a slave will be described first of all as is shown in FIG.2, in the form of a timing diagram with counter values n plotted againsttime t. Each slave has a timer which is set to a specific value (Nsync)by the reception of a specifically defined slave-specific message (forexample a synchronization message which may also transport user data)from the master relating to the synchronization time (Tsync). This valueis calculated in advance by the master and is reported together with theoverall cycle time (Tcycl) to the slave in an initialization phase. Thismethod allows any possible brief failure of this slave-specificsynchronization message to be bridged, since the counter starts a newcycle once the overall cycle time (Tcycl) has elapsed, even without anyneed to be corrected in the meantime by means of the synchronizationmessage.

This is seen in FIG. 2, where the profile of a clock signal n with theoverall cycle time (Tcycl) is plotted against time t. If thesynchronization signal for the slave now fails for a number of cycles,then the timer continues to run autonomously. In this case,discrepancies can occur between the overall cycle clock (Tcycl) and thetimer-internal clock, for example due to crystal drift in the counter.Such a clock error is shown in the form of a dotted line. Any error Δwhich is present at the synchronization time (Tsync) can be correctedwhen a synchronization signal is once again available, and this resetsthe timer.

The crystal drift between a master and slave (for example ±100 ppm,crystal characteristic) together with the cycle time (Tcycl) of thecyclic data traffic governs the accuracy limits of the timeslots foraccess control. In this case:

-   -   cycle time×crystal drift>maximum accuracy of the timeslot        (example: 1 ms×±200 ppm=±0.2 μs>maximum accuracy of the        timeslot).

Each communication network KOMSYS1, KOMSYS2 of network subscribersconsists of one, and only one, master unit and one or more slave unitsand, according to the invention, is operated for cyclic data interchangeusing a timeslot access method, in order to achieve optimum dead-timedata transmission.

In an initialization phase, in which only the master may transmit andonly the slaves may respond, each slave receives the information on thetimes at which it will receive messages from the master and on the timesat which it can send its message or its messages. The precise definitionof the times at which the messages must be sent or received allows thecommunication to be controlled both at the master end and at the slaveend. FIG. 3 shows the sequence of such a timeslot access methodaccording to the present invention.

Specifically, FIG. 3 shows the access of a master unit MS to two slaveunits SL1 and SL2 within one overall cycle (Cycl) or (Tcycl). Once afirst time Tsend1 has elapsed and after the start of the cycle, themaster MS sends a transmitted message Ssl1 to the slave SL1. Owing tothe delay times on the communication path, this message arrives at theslave unit SL1 as a received message Rsl1 with a certain time delaydtDel(sl1).

The interval between the end of the received message and the startingpoint of the respective overall cycle (Cycl) represents the receptiontime Trec(sl1) for the slave SL1, which corresponds to the specificsynchronization signal Tsync1 for the slave SL1. At this time, the slaveSL1 always receives its synchronization signal and its data. This timerepresents the clock time for the slave unit SL1, and its timer is setto a value corresponding to the time Tsync1. The slave SL1 is thusassigned the timeslot associated with this for reception of messages.The timer is, for example, decremented, with the set value beingdesigned such that the counter zero crossing is coincident with the endof the overall cycle. The slave SL1 now sends its response message Sms1to the master, to be precise exactly after a transmission timeTsend(sl1) for the slave SL1, measured from the start time of therespective overall cycle. This transmission time Tsend(sl1) is allocatedin a fixed manner to the slave SL1, and represents its timeslot forsending messages. This message once again arrives with the delaydtDEL(Sl1) which is governed by the delay time in the communicationpath, as the received message Rms1 at the master unit MS. The timeinterval from the start time of the respective overall cycle (Cycl) andthe end of the received message Rms1 represents the reception time Trec1for the response from the slave SL1 from the point of view of the masterMS.

The communication between the master MS and the second slave unit SL2takes place on the basis of the same fundamental procedure. Once afurther time Tsend2 has elapsed after the cycle start, the master MSsends a further transmitted message Ssl2 to the slave SL2. Owing to thedelay times in the network, this message once again arrives as thereceived message Rsl2 at the slave unit SL2 with a certain time delaydtDel(sl2).

The interval between the end of the received message and the start timeof the respective overall cycle (Cycl) represents the reception timeTrec(sl2) for the slave SL2. This corresponds to the specificsynchronization signal Tsync2 for the slave SL2. The slave SL2 alwaysreceives its synchronization signal and its data at this time. This timethus represents the clock time for the slave unit SL2 in a correspondingmanner, and its timer is set to a value corresponding to the timeTsync2.

The slave SL2 is thus assigned the timeslot associated with this forreceiving messages. The timer is, for example, likewise decremented,with the set value being designed such that the counter zero crossing iscoincident with the end of the overall cycle. The slave SL2 sends itsresponse message Sms2 to the master MS, to be precise exactly after atransmission time Tsend(sl2) for the slave SL2, which is measured fromthe start time of the respective overall cycle. This transmission timeTsend(sl2) is allocated in a fixed manner to the slave SL2 andrepresents the timeslot for sending messages. The slave unit SL2 cantherefore start to send a message Sms2 to the master MS only beforecomplete reception of the message Rsl2, since the corresponding timeslot for sending after Tsend2 is already known from a previous period ofthe overall cycle (Cycl), and the slave unit SL2 is thus already in thesteady synchronization state.

This message also arrives with the delay dtDEL(sl2), which is governedby the delay time on the communication path, as the received messageRms2 at the master unit MS. The time interval from the start time of therespective overall cycle (Cycl) and the end of the received message Rms2represents the reception time Trec2 for the response from the slave SL2from the point of view of the master MS.

The procedure for the other slave units likewise correspond with eachslave unit being allocated its own exclusive timeslot. Hence, it ispossible to immediately identify not only interference, but also delayedor premature transmission of a message. A specific message to a slave isused for synchronization in addition to data transmission.

Apart from timeslot control, the synchronization also means that theactual values can be stored in all the slaves at a specific common time,in this case the start of the cycle (Cycl), in order to achievesimultaneous, equidistant sampling for the controller R. To this end,the respective actual value from each slave unit, in particular from thesensors and transmitter systems S1 and S2, is stored when thecorresponding timer reaches its zero crossing, and is then transmittedto the higher-level master unit NC or R. In the case of the drive systemwith two coupled axes shown in FIG. 1, the slave units S1, S2 supplycorresponding rotation speed, position and orientation actual values.

FIG. 4 shows one possible message frame for a message T to beinterchanged via the network. The message starts with a message preamblePR, the length of which is, for example, 64 bits. This is followed by afunction code sequence FCS of 16 bits, which is explained in thefollowing text with reference to FIG. 5. This is followed by 8 bits withthe address SLA of the slave unit. In this case, the number of bitsrequired depends on the number of slave units to be addressed. If thereare 8 bits, the number of slaves which can be addressed is 255 (thevalue ZERO is normally reserved for a broadcast function). This isfollowed by the length LE of the subsequent user data DU. The messagelength is thus variable. The message frame ends with a checksum CRC witha length, for example, of 32 bits (CRC stands for cyclic redundancycheck, a known method for error monitoring).

FIG. 5 shows one possible detailed view of the section comprising thefunction code sequence FCS. The most significant bit contains asupervisor bit SV. This represents a sign of life such that, for exampleif the control software crashes, it is no longer set in the master. Thisallows, for example, a change to a safe state to be initiated in theslave.

The bits 14 to 10 are reserved for any desired tasks. They are followedby three control bits CTRL0 to CTRL2, an area of three to six bits forstoring the message type TY and a further bit LI which signals whetherany length information is present or whether a fixed, standardizedmessage length can be assumed. The bit M/S signals whether this is amessage which is being transported from a master to a slave or viceversa, and thus indicates the transport direction. The last bit SYNCindicates whether this is a synchronization message or a user message.It is also feasible for the SYNC bit to be used to signal that themessage contains additional synchronization data as well as user data.

The control bits CTRL0 to CTRL2 are used, for example, to providespecific security or safety functions, such as those which are required,in particular, in the field of industry automation. One possibleimplementation of a security or safety function with the aid of thecontrol bits CTRL0 to CTRL2 is shown in FIG. 6 which shows a blockdiagram with the internal construction of a slave unit, for example ofthe power section A1, from the drive controller shown in FIG. 1.

If the module Kom, which receives the message via the line driver PHYand implements the communication protocol, is independent of amicroprocessor μP in the slave application (the actual power section L),specific application events can be initiated in the slave A1 by means ofthe control bits CTRL0 to CTRL2 without requiring the microprocessor μPand the corresponding software in the slave A1. This corresponds to asecond initiation channel K2 as is required for certain security andsafety applications (for example emergency stop etc.).

The above method steps according to the invention allow a communicationnetwork with a deterministic time response, and hence the capability forreal-time communication, to be achieved, in particular for industryautomation as well, on the basis of Ethernet physics.

1. A method for real-time communication between a number of networksubscribers in a communication system using Ethernet physics, comprisingthe steps of transmitting messages via Fast-Ethernet devices toestablish communication between a master unit and one or more slaveunits with one another, synchronizing the master unit and the one ormore slave units by means of a common timebase to interchange messagescyclically within a total cycle time, and assigning each slave unit afirst timeslot within said total cycle time for transmission of atelegram and a second timeslot for reception of a telegram; andassigning the master unit a third timeslot within said total cycle timefor transmission of a telegram and a fourth timeslot for reception of atelegram, wherein each slave unit being timed by way of a respectivecounter with a preassigned total cycle time, the respective counterbeing set cyclically by reception of respective slave-specificsynchronization information determined by the master unit and eachcounter of a slave unit, even in the absence of the respectivesynchronization information after expiration of the preassigned totalcycle time automatically starting a new cycle.
 2. A method according toclaim 1 wherein the synchronization information comprises a respectivesynchronization time and an associated number value assigned for eachslave unit.
 3. A method according to claim 2, wherein the respectivesynchronization information, the total cycle time, the timeslots areassigned to each slave during an initialization phase.
 4. A methodaccording to claim 1, wherein current instantaneous values are stored ineach slave unit at a common point of time.
 5. A method according toclaim 1, wherein each slave unit in each telegram sends a signal to themaster unit and the master unit, in the absence of said signal,controlledly stops the corresponding slave unit.
 6. A method accordingto claim 1, wherein each slave unit with each telegram receives controlinformation from the master unit with which, by way of a secondtriggering channel, safety-oriented functions provided can be activateddirectly in the slave unit.
 7. A method according to claim 1, whereineach slave unit with each telegram receives from the master unit amaster sign-of-life signal, and each slave unit in the absence of saidsignal automatically stopping itself.
 8. A method according to claim 1,wherein separate transmission and reception lines between two networksubscribers are used simultaneously, in that all slave units willtransmit only in the direction towards the master unit, and receivetelegrams only from the master unit from the master direction.
 9. Amethod of real time communication between network subscribers to severalcommunication systems with Ethernet physics, wherein a majority ofnetwork subscribers having a circuit part to form network nodes, servingto pass along the telegrams towards another master unit or additionalslave units, wherein the network subscribers communicating with eachother directly within each communication system or via a network nodeaccording to claim
 1. 10. A distributed drive system with hierarchicalnetwork for real-time communication between a number of communicationsystems using Ethernet physics, the distributed drive system comprisinga first communication system including a numeric motion control asmaster unit and at least one regulating unit as slave unit, eachregulating unit serving as master unit of an additional communicationsystem comprising at least one power part to trigger a motor and anassociated emitter system as slave units, the system further comprising:means for transmitting messages via Fast-Ethernet devices to establishcommunication between a master unit and one or more slave units with oneanother, means for synchronizing the master unit and the one or moreslave units by means of a common timebase to interchange messagescyclically within a total cycle time, means for assigning each slaveunit a first timeslot within said total cycle time for transmission of atelegram and a second timeslot for reception of a telegram; and meansfor assigning the master unit a third timeslot within said total cycletime for transmission of a telegram and a fourth timeslot for receptionof a telegrams, wherein each slave unit comprises a counter for timing apreassigned total cycle time, the respective counter being setcyclically by reception of respective slave-specific synchronizationinformation determined by the master unit and each counter of a slaveunit, even in the absence of the respective synchronization informationafter expiration of the preassigned total cycle time automaticallystarting a new cycle.
 11. A system for real-time communication between anumber of network subscribers in a communication system using Ethernetphysics, comprising: means for transmitting messages via Fast-Ethernetdevices to establish communication between a master unit and one or moreslave units with one another, means for synchronizing the master unitand the one or more slave units by means of a common timebase tointerchange messages cyclically within a total cycle time, means forassigning each slave unit a first timeslot within said total cycle timefor transmission of a telegram and a second timeslot for reception of atelegram; and means for assigning the master unit a third timeslotwithin said total cycle time for transmission of a telegram and a fourthtimeslot for reception of a telegram, wherein each slave unit isoperable to receive with each telegram from the master unit a mastersign-of-life signal, and each slave unit comprises means which in theabsence of said signal automatically stop the slave unit.
 12. A methodfor real-time communication between a number of network subscribers in acommunication system using Ethernet physics, comprising the steps oftransmitting messages via Fast-Ethernet devices to establishcommunication between a master unit and one or more slave units with oneanother, synchronizing the master unit and the one or more slave unitsby means of a common timebase to interchange messages cyclically withina total cycle time, and assigning each slave unit a first timeslotwithin said total cycle time for transmission of a telegram and a secondtimeslot for reception of a telegram; and assigning the master unit athird timeslot within said total cycle time for transmission of atelegram and a fourth timeslot for reception of a telegram, wherein eachslave unit with each telegram receives control information from themaster unit with which, by way of a second triggering channel,safety-oriented functions provided can be activated directly in theslave unit.
 13. A method for real-time communication between a number ofnetwork subscribers in a communication system using Ethernet physics,comprising the steps of transmitting messages via Fast-Ethernet devicesto establish communication between a master unit and one or more slaveunits with one another, synchronizing the master unit and the one ormore slave units by means of a common timebase to interchange messagescyclically within a total cycle time, and assigning each slave unit afirst timeslot within said total cycle time for transmission of atelegram and a second timeslot for reception of a telegram; andassigning the master unit a third timeslot within said total cycle timefor transmission of a telegram and a fourth timeslot for reception of atelegram, wherein each slave unit with each telegram receives from themaster unit a master sign-of-life signal, and each slave unit in theabsence of said signal automatically stopping itself.